The World has been blessed with the capability of providing various products that benefit mankind such as foods for human consumption, beverages, animal feeds, building materials, and the like. A downside to this blessing is the fact that, in the manufacture of these materials, large amounts of waste are created, and typically, these wastes are burned or buried in order to dispose of them. It now becomes environmentally and economically necessary to consider ways in which these wastes can be removed and at the same time, create products or byproducts that can be recycled back into commerce or can otherwise be beneficial to mankind.
For economical and environmental reasons, recent studies focus on utilization of natural materials, either in the natural state, or as a waste stream from the processing of natural materials, as renewable, versatile, biodegradable resources for the production of novel materials. Polysaccharides are major constituents of these natural materials. With the term “polysaccharide” it is meant a pure carbohydrate, such as starch or cellulose, whereas “polysaccharide-based material” refers to materials containing a polysaccharide as major component, such as pulp, the major component of which is alpha-cellulose. Hereinafter both alternatives will be meant, if not stated otherwise, when using the wording “polysaccharide”. Polysaccharides are increasingly used as a source of raw material for the chemical industry whereby they are converted to useful products. Examples of such materials include the processing of novel materials from wood cellulose, hemicelluloses of straw, grass, leaves, fruits and vegetables, and starch of cereals and tubers.
Several attempts have been made to utilize polysaccharides for the formation of biodegradable plastics. For example, Albertson and Ranby describe the formation of polyethylene foils blended with starch granules sealed inside the polyethylene microstructure. A. C. Albertson and B. Ranby, (1979), J. Appl. Polym. Sci., 35, 413-430. However, this method has been nearly completely abandoned because only the starch component biodegraded leaving powdered polyethylene behind in the environment.
U.S. Pat. No. 5,852,114, issued Dec. 22, 1998 to Loomis, et al., describes a biodegradable thermoplastic polymer blend in which a first polymer and a second polymer are intimately associated together in a uniform, substantially homogeneous blend. The composition may further comprise a polysaccharide component such as starch.
U.S. Pat. No. 5,166,336, issued Nov. 24, 1992 to Yamauchi, et al., describes a process for producing a corn milling residue carboxymethylether salt comprising reacting a corn milling residue with alkali in the presence of an aqueous carboxymethylating agent solution to give a corn milling residue carboxymethylether salt with an average degree of substitution of not less than 0.2.
U.S. Pat. No. 6,103,885, issued Aug. 15, 2000 to Batelaan, et al, describes a process for the amidation of a material having at least one carboxyl-containing polysaccharide. The carboxy groups are reacted with an ammonium donor of the general formula —NH to form the corresponding polysaccharide carboxyl ammonium salt, and a second step in which the polysaccharide carboxy ammonium salt is heated so as to convert the ammonium groups into corresponding amido groups.
U.S. Pat. No. 7,225,732, issued on Aug. 14, 2007 to Fischer et al, describes a process to solve the problem that starch alone is too viscous and intractable for conventional thermoplastic molding equipment such as injection molding or compound extrusion. The invention entails formation of a thermoplastic composition made of a mixture of starch and dialdehyde polysaccharide. The latter is produced from the chemical reaction set forth in FIG. 1, wherein periodate oxidation of starch cleaves the C2-C3 bond of the anhydroglucose unit of the polysaccharide to produce dialdehyde polysaccharide.
Additionally, there has been reported the production of biodegradable plastics from proteins. J. Jane and S. T. Tim, (1995). Progress in Plant Polymeric Carbohydrate Research (eds. F. Meuser, D. J. Munnes, and W. Seibel), Behr's Verlag, Hamburg, pp. 165-168; C. H. Schilling, T. Babcock, S. Wang, and J. Jane, (1995). J. Mater. Res. 10, 2197-2202 and J. Zhang, P. Mungara, P., and J. Jane, (2000). Polymer, 42, 2569-2578.
U.S. Pat. No. 5,710,190, issued Jan. 20, 1998 to Jane, et al., describes a biodegradable thermoplastic composite made of soy protein, a plasticizing agent, a foaming agent, and water that can be molded into biodegradable articles that have a foamed structure and are water-resistant with a high level of physical strength and/or thermal insulating properties.
Additionally, there has been reported the production of biodegradable plastics from mixture of polysaccharides and proteins. For example, U.S. Pat. No. 5,593,625, issued Jan. 14, 1997 to Reibel et al., describes a method of heating blends of legume-based thermoplastic materials (proteins) and cellulose fibers to form hard composite material. The composite microstructure consists of cellulose fibers embedded in a protein matrix. The fibers and matrix are chemically bonded to each other by hydrogen bonds that are naturally present in both cellulose and protein.
Biodegradable plastics have been made from several protein/anionic polysaccharide reaction products that were synthesized by an electrochemical method. For example, potato starch, pectin, xanthan gum, carrageen, and carboxymethyl cellulose served as anionic polysaccharide components that were used in forming reaction products with proteins. A. Dejewska, J. Mazurkiewicz, P. Tomasik, and H. Zaleska, (1995) Staerke, 47, among others.
U.S. Pat. No. 5,397,834, issued on Mar. 14, 1995 to Jane et al., describes a biodegradable, thermoplastic composition made of a two-step process of (1) periodate oxidation of starch to form dialdehyde starch by the same reaction described earlier in U.S. Pat. No. 7,225,732 and shown in FIG. 1; and (2) reaction of dialdehyde starch with free amino groups on proteins, a reaction that entails imine formation and subsequent Mannich reactions, yielding strong, covalently bonded cross linked products. In step two, the aldehydro groups of dialdehyde starch chemically bind with the amino groups of proteins forming CH═NH— moieties characteristic for Schiff bases. In turn, the resulting strong, covalent bonding between proteins and starch plays a significant role in improving tensile strength and slowing biodegradation; the greater the concentration of strong interpolymer covalent bonds between protein and starch, the greater the tensile strength and the slower the rate of biodegradation. It should be noted that step one of the process requires the use of expensive periodate reactants.
There is also disclosed in U.S. Pat. No. 5,523,293, that issued Jun. 4, 1996 to Jane, et al., and U.S. Pat. No. 5,665,152 that issued Sep. 9, 1997 to Bassi et al., a biodegradable thermoplastic polymer blend in which soybean protein is reacted with a carbohydrate filler, a plasticizer, and a reducing agent. Both of these patents entail use of a reducing agent to solve the problem that protein alone is too viscous to be molded into solid plastic articles by conventional thermoplastic molding equipment. The reducing agent is specifically designed to produce compositions with reduced viscosity by cleaving disulfide bonds in protein, thereby reducing the protein viscosity. The reducing agent is selected from the group consisting of sodium sulfite, sodium bisulfite, potassium sulfite, potassium bisulfite, sodium nitrite, sodium hydrosulfite, sodium pyrosulfite, ammonium sulfite, mercaptoethanol, cysteine, L-cysteine hydrochloride, cysteamine, L-cysteine tartrate, di-L-cyteine sulfite, ascorbic acid, hydrogen sulfide, glutathione, and combinations thereof. It should be noted that use of these reducing agents is expensive. It should also be noted that the main difference between U.S. Pat. No. 5,523,293 and 5,665,152 is that the latter places an upper limit of 80 degrees Centigrade on the thermoplastic forming temperature in order to avoid heat denaturation of protein.
At a microscopic scale within protein—polysaccharide composite materials, the extent of interpolymeric chemical bonding (crosslinking) between protein and polysaccharide molecules plays a significant role in three essential aspects of materials manufacturing and physical properties: (i) interpolymeric protein—polysaccharide bonds break and reform during viscous deformation of the composite material thereby controlling the viscoelastic properties of the composite material during plastic thermoforming operations; (ii) the same interpolymeric bonds between protein and polysaccharide molecules control the tensile strength and ductility of the molded composite material; and (iii) the same interpolymeric bonds between protein and polysaccharide molecules influence the rate of biodegradation of the molded composite material. With the phrase, “the extent of interpolymeric chemical bonding (crosslinking) between protein and polysaccharide molecules,” it is meant both (i) the bond energies of individual interpolymeric bonds between protein and polysaccharide molecules in the composite material and (ii) the number density of interpolymeric bonds between protein and polysaccharide molecules per unit volume of the composite material. From hereinafter, the phrase, “extent of interpolymeric chemical bonding (crosslinking) between protein and polysaccharide molecules” is meant both (i) the magnitudes of the bond energies of individual interpolymeric bonds between protein and polysaccharide molecules in the composite material and (ii) the number density of interpolymeric bonds between protein and polysaccharide molecules per unit volume of the composite material.
It is important to distinguish differences in the extent of interpolymeric chemical bonding (crosslinking) between protein and polysaccharide molecules in the prior art. Earlier it was mentioned that U.S. Pat. No. 5,397,834 describes a composite material where dialdehyde starch is produced by periodate oxidation; thereafter, dialdehyde starch is covalently bonded to protein molecules via —CH═NH—moieties. It naturally follows that these strong, covalent, interpolymeric bonds between protein and starch significantly influence (i) the viscoelastic properties of the composite material during thermoplastic molding, (ii) the mechanical properties of the molded composite material, and (iii) the rate of biodegradation of the molded composite material. Earlier it was also mentioned that U.S. Pat. Nos. 5,523,293 and 5,665,152 describe protein—starch composites made by a chemical process that cleaves disulfide bonds in protein. In the microstructure of those composite materials, protein and polysaccharide molecules are cross linked by weak intermolecular hydrogen bonds. It naturally follows that these weak intermolecular hydrogen bonds between protein and polysaccharide molecules significantly influence (i) the viscoelastic properties of the composite material during thermoplastic molding operations, (ii) the mechanical properties of the molded composite material, and (iii) the rate of biodegradation of the molded composite material.
The object of this invention disclosed herein is to develop an improved protein-polysaccharide composite material with improved viscoelastic properties during thermoforming, improved mechanical properties in the formed composite and improved biodegradation characteristics without the need to chemically modify the protein or to break the C2-C3 bond of the anhydroglucose unit of the polysaccharide, both of which require expensive chemical treatments. The object is solved by a unique approach to interpolymeric chemical bonding (crosslinking) between protein and polysaccharide molecules: electrostatic chemical bonds (ionic bonds or salt bridges).
The object is solved by a five-step process of (i) dispersing the polysaccharide in an aqueous basic solution, (ii) agitating the mixture for a period of time, (iii) subsequently treating the mixture with a carboxylating process, where the phrase, “carboxylating process” hereinafter means any process or agent that results in the conversion of any hydroxyl group on any carbon atom of the anhydroglucose unit of the polysaccharide to a carboxylate, thereby forming a polysaccharide carboxylate (corresponding to FIG. 2), (iv) subsequently reacting the mixture with protein molecules to crosslink proteins to polysaccharide carboxylate molecules by electrostatic bonds (i.e., ionic bonds or salt bridges corresponding to FIG. 3), and (v) drying the composite material with a drying technique such as air drying, oven drying, spray drying, supercritical CO2 drying, solvent dehydration, or a combination thereof. The resulting liquid material in step (iv) can be molded into a shaped component and dried into a final, solid shape. Alternatively, the dried material in step (v) can be thermoplastically molded by conventional plastic molding equipment.
Examples of reactants for the carboxylating process in this invention include but are not limited to reagents such as hypochlorite, acylating agents, and carboxymethylating agents. Hypochlorite and similar agents convert primary alcohols (e.g., C6 of the anhydroglucose unit of the polysaccharide) to carboxylates, whereas acylating agents, such as anhydrides, create an ester with the carboxyl function. Further, carboxymethylation with reagents such as chloroacetic acid forms an ether bond with the carboxyl function. The latter is exemplified by the well-known commercial product carboxymethylcellulose. The carboxylating process does not include periodate oxidation, which produces only dialdehyde polysaccharide, which is generally known in the field and does not produce carboxylates through the chemistries in this patent.
An alternate embodiment is to form the resulting liquid materials in step (iv) into thin sheets by conventional papermaking technology, where the thin sheets are formed by drainage of a fibrous-material suspension on a screen or between two continuously revolving screens. Alternatively, prior to advancing to step (v) in the manufacturing process, the liquid materials produced in step (iv) can also be applied as an adhesive film to bond various materials together including paper, wood, metal, glass, and ceramics.
It is important to note that crosslinking of polysaccharide carboxylates to protein in step (iii) entails reacting the carboxy groups of the anhydroglucose unit of the polysaccharide chain with protein, thereby forming electrostatic chemical bonds (ionic bonds or salt bridges) between protein and polysaccharide molecules (corresponding to FIG. 3). Such crosslinking does not entail the formation of strong, covalent bonds between protein and polysaccharide molecules. The entire process does not entail cleavage of any C—C bonds of the anhydroglucose units of the polysaccharide chain.
In addition, while U.S. Pat. Nos. 5,523,293 and 5,665,152 emphasize the need to break disulfide bonds in protein to allow flexibility in the final mixed bioplastic, we obtain a similar flexibility with less expensive reagents by making the chemical bond between the protein and polysaccharide itself flexible by producing the ionic bond. These bonds more easily break and reform under plasticizing conditions without the need to chemically modify the protein. Thus, the extrusion and molding properties of our approach come from entirely different chemistries.
It is expected that the ionic bond between amine groups of the protein and the polysaccharide carboxylate has an order of magnitude higher bond energy than a hydrogen bond. Hence, the invention accomplishes a strong, mixed copolymer with a lower intermolecular bond density than with simple intermolecular hydrogen bonding. This bond strength weakens in aqueous suspensions, but forms a stronger bond as the water is removed in the process of filtration or dehydration. Moreover, the invention likely has a similar intermolecular bond density as that described previously in U.S. Pat. No. 5,397,834, where dialdehyde starch is covalently bonded to protein molecules via —CH═NH—moieties. However, the ionic interpolymeric bond between protein and polysaccharide carboxylate in the invention is more easily metabolized during biodegradation than the strong, covalent bond crosslinking aldehyde starch to protein in U.S. Pat. No. 5,397,834. As a result, it is likely that the protein-polysaccharide carboxylate materials in the invention described herein have different biodegradation characteristics than that of the protein—aldehyde starch materials of U.S. Pat. No. 5,397,834.
A second objective of the invention described herein is to provide a robust manufacturing process capable of accepting a wide range of feedstock raw materials, including common polysaccharides that are currently in abundance as industrial waste materials or by-products. Examples of such raw materials are described below and include ethanol distillers' grain, sugar beet pulp, sawdust, and corncob.
Distiller's grain is a by-product or waste product from the manufacture of ethanol from crops including corn. Significant growth in the worldwide production of distiller's grain is anticipated as a result of rapid growth in the mass production of corn-derived ethanol for transportation fuel. Currently, the main use of this material is as an animal feed. It can also be incorporated into human snack food and spaghetti, and in one instance, it has been reported as an extender and thickener in urea-formaldehyde plywood adhesives.
Also, corncobs are usually considered waste material from industrial utilization of maize crops. Several applications of corncobs have been reported in the literature. For example, pulverized corncobs were admixed with various glues and petroleum-derived fibers to produce lignocellulosic composites. Polypropylene and other engineering polymers have been reinforced with pulverized corncob fiber and attempts to use shredded corncobs in paper making have also been published.
Corncobs, being largely cellulose and hemicellulose possess excellent absorbing properties and have been used in a variety of applications as absorbents, animal bedding, stove and furnace fuel, and as a carrier of agricultural fertilizers. They have also been transformed to charcoal and subsequently used as a sorbent. Pyrolysis of corncobs results in the production of furaldehyde and acetic acid. Enzymatic treatment of corncobs provides acetone and butanol as well as D-xylan and D-xylose. They are commonly pulverized into fine powder particles that are subsequently used as industrial abrasives. Corncobs contain approximately 47% cellulose in their woody fraction, and 36% cellulose in the pith and chaff fraction. In both fractions, approximately 37% hemicelluloses and 35 to 36% pentosans exist.
There should also be considered sawdust, which is a voluminous waste material of the forest products industry. Several value-added applications of sawdust have been reported in the literature. For example, the production of solid fuel by briquetting or pelletizing sawdust is common. Co-fermentation of sawdust with manure and co-liquefaction with coal are alternative routes to energy production. The use of sawdust as construction material for wood product boards and panels has been known since the nineteenth century. Recent developments include the use of sawdust for reinforcing polymers and as a component of wood-based cement-bonded boards.
There are many other natural sources of polysaccharides including leaves, bark, roots, straw, shells of seeds, stems of plants, and especially sugar beet pulp as a large volume by-product from the production of sugar from sugar beets. Although a large amount of this pulp is utilized as animal feed, the production of L-arabinose, and the production of paper, this utilization is not enough to significantly reduce the amount of this by-product.